1. Field of the Invention
The present invention relates, in general, to visual display signs, and, more particularly, to a lighting unit for a low-cost visual display sign using incandescent bulbs.
2. Statement of the Problem
People have long used outdoor signs for advertising, sending messages to others, and displaying artwork. In early times, carved and painted wooden signs were hung outside of taverns and inns to advertise their presence and accessibility for travelers. In more recent times, large billboards are erected near highways and messages are painted on the sides of barns or other large buildings to capture the attention of passing motorists. Such large billboards present an advertisement or message in a static form. That is, the billboard is most often painted or has a previously prepared advertisement attached to it. The message remains on the sign until it is desired to replace the message with a different communication. These billboards are often illuminated during the hours of darkness in order to reach even more viewers. Such illumination is usually achieved with incandescent or other very bright lights.
Many advertisers, however, desire to change their messages more often than is possible with a billboard that must be repainted each time the message is changed. Advertisers also desire to prepare their own unique messages or to use a moving display as opposed to a static picture. Thus, a need exists for large visual display signs showing moving images that can be located outdoors at remote locations such as on a building or a freestanding pylon.
Although visual display systems exist providing moving images in color, for example, a television show or videotape, such display signs have not provided vivid video images, especially those that are used outdoors. Previous signs designed for outdoor use have been prohibitively expensive to operate, to purchase, and to maintain. Lack of brightness is a serious problem for prior outdoor display signs. Light from other sources, especially the sun and the moon, but also including, but not limited to, other lights on the ground or on adjacent buildings, reduces the ability of viewers to perceive the picture displayed by the sign. These outdoor display signs are comprised of a plurality of lighting units, some of which are turned off, and therefore black, at any particular moment, and some of which are lit in order to show the picture. Ambient light reduces the contrast of the sign in two ways. First, when a lighting unit is off, the blackness of the lighting unit is only as dark as the ambient light. Second, when a lighting unit is lit, its colors mix with the ambient light. Thus, external light washes out the picture, causing the viewer to have difficulty perceiving the picture being displayed, especially when that picture is a moving picture in color. Thus, a need exists for a lighting unit that will increase the light output of a visual display sign, without using expensive components.
Prior cooling systems for lighting units do not solve the problems of cooling outdoor display signs that comprise thousands of lighting units. Cooling systems that use interior convection currents and radiation through the housing to dissipate the heat of a lighting unit are inefficient at reducing the heat generated by a high-intensity bulb. A system in which the heat is radiated by the housing into the surrounding atmosphere is also difficult to use in a display sign having many thousands of closely apposed lighting units attached to a backing. If a radiant cooling system is used in such signs, the heat produced by the many lighting units can be radiated only through the limited area of the light-emitting end of the unit.
In other systems, pressurized air motivated by, for example, a fan moves along the interior of the reflector and the exterior of the light bulb. After a time using such a cooling system, dust particles in the air (despite the use of air filters) settle on and cover the reflector, the bulb, and the inside of the lens, if any, decreasing the amount of light emitted by the lighting unit and causing the picture displayed by the sign to become dimmer. Such dust buildup also brings about a need for frequent cleaning of the lighting unit, causing expensive maintenance for the advertiser or other user of the sign, and providing opportunities for costly breakage during the cleaning process. Thus, a need exists for an efficient cooling system for a lighting unit that does not cause dust buildup on the light-emitting surfaces. To achieve this, the lighting unit of the present invention is substantially sealed. A patentability search for sealed lighting units with cooling systems resulted in the following patents:
U.S. Pat. No. 1,245,426 to Bennett teaches a non-glaring headlight having a light source and a reflector held within a casing. A lens is placed over the open end of the reflector. No cooling system is disclosed in this patent.
U.S. Pat. No. 2,282,587 (the '587 patent) to Kurth teaches a ventilating and illuminating device having an enclosed globe containing an electric lamp and surrounded by a series of nested hollow flaring members. The flaring members are spaced apart from each other and from the globe, and air is able to pass through the spaces from or to an air supply duct attached to the rear of the outermost flaring member. No air passes within the globe. In this invention the flaring members have open ends, whereas the globe is sealed with a solid lid.
U.S. Pat. No. 3,310,672 (the '672 patent) to Bursell discloses a combined lighting fixture and air intake in which one embodiment has a casing with slots at the top, and the slots have tongues that deflect the pressurized air supply flowing into the slots and set the air into a whirling motion. Inside the casing and spaced away from the casing is a cylindrical wall, and sealed within the cylinder formed by the wall is a lighting unit with a reflector. The open end of the reflector is not sealed; however, the open end of the cylinder is sealed by a solid anti-dazzling device. The pressurized air enters the slots, is deflected by the tongues, and with a helical motion around the cylinder travels from the slots through the space between the casing and the cylinder and exits at the open end at the bottom of the casing.
U.S. Pat. No. 3,639,751 (the '751 patent) to Pichel discloses a thermally dissipative enclosure for a high-intensity illuminating device. In one embodiment, this patent teaches a closed wall forming a housing cavity having an open mouth and a hollow base. A light source is placed within the base and extends into the cavity. A reflector is positioned within the cavity and spaced apart from the wall of the cavity to form a surrounding channel between the wall of the cavity and the reflector. The housing cavity may have protrusions extending between the wall and the reflector to provide points of attachment for the reflector. The open end of the cavity is sealed with a faceplate. The open end of the reflector is not sealed. Thermal energy generated by the operation of the light source is transferred to the housing by convection via the channel surrounding the reflector, and the housing then radiates the heat to the outer atmosphere.
In addition to their difficulties in seeing moving pictures on outdoor display signs under lighting conditions that wash out the light emitted from the sign, viewers on the ground often find it difficult to see the picture on a visual display sign when they are viewing the sign from an angle other than directly in front of the sign, for instance, if they are standing below and close to the sign or passing by the side of the sign, because most of the light emitted from the lighting units is directed horizontally from the front of the sign or is diffused too far around the edges of the lighting units. Thus, a need exists for a lighting unit that directs the emitted light downward toward viewers below and in front of the visual display sign and in which the reflector is designed to emit light at a predetermined angle. A patentability search for angled lighting units and for reflectors emitting light at a predetermined angle resulted in the following patents:
U.S. Pat. No. 5,400,228 to Kao discloses a full-color illuminating unit including a circuit board and a plurality of red and green lights disposed around a central blue light. A protrusion above the illuminating unit acts as a shade to block sunlight. The entire illuminating unit is positioned at a downward angle against a vertical backing so that the light emitted from the illuminating unit can project to the ground in an appropriate angle (see FIG. 3).
U.S. Pat. No. 5,410,328 to Yoksza et al. teaches a display comprising a plurality of LED modules. Each module is attached to the vertical wall of the display is such a manner that the module angles downward (see FIG. 2A).
U.S. Pat. No. 4,504,894 to Reibling discloses a reflective surface contoured to control the angle of emanation of the light rays from the lighting unit, with a cutoff of 65 degrees from the vertical axis.
A need therefore exists for an affordable lighting unit for use in visual display signs that provide highly visible moving pictures at the NTSC standard 30 images per second both indoors and outdoors under all lighting conditions. Such a lighting unit should provide a highly efficient light output that is directed at an angle to the ground, so that the pictures displayed can be easily seen by viewers below the sign. In addition, a need exists for a method to efficiently cool the lighting units of the visual display sign without allowing dust to reach and cover the light-emitting portions of the lighting unit.